1. Field of the Invention
The invention relates to a method and to a tool in each case for generating a thread in a workpiece.
2. Background
For thread generation or thread reworking, both cutting and also non-cutting processes and threading tools are known. Cutting thread generation is based on material removal from the workpiece in the region of the thread flight. Non-cutting thread generation is based on deformation of the workpiece and generation of the thread flight in the workpiece by pressure. An overview of thread generating tools and working methods in use is given in Handbuch der Gewindetechnik and Frästechnik [Manual of threading practice and milling practice], publisher: EMUGE-FRANKEN, publishing firm: Publicis Corporate Publishing, year of publication: 2004 (ISBN 3-89578-232-7), designated below only as “EMUGE manual”.
Falling within the scope of cutting thread generation are taps (cf. EMUGE manual, chapter 8, pages 181 to 298) and thread milling cutters (cf. EMUGE manual, chapter 10, pages 325 to 372) and also, only for external threads, thread-cutting dies (cf. EMUGE manual, chapter 11, pages 373 to 404).
A tap is a thread-cutting tool whose cutting edges or thread-cutting teeth are arranged along an external thread beneath the thread pitch of the thread to be generated. During the generation of the thread, the tap is moved with a feed motion axially with respect to the tool axis, and whilst being rotated about its tool axis with a rotational speed which is dependent on the axial feed speed in accordance with the thread pitch, into a cylindrical core hole in a workpiece, wherein the tool axis of the tap is oriented coaxially with respect to the central axis of the core hole, and the cutting edges of said tap are permanently in engagement with the workpiece at the core hole wall (continuous cutting), such that a continuous thread flight is generated on the core hole wall.
Falling within the scope of non-cutting thread generating tools are so-called cold-forming taps (cf. EMUGE manual, chapter 9, pages 299 to 324) and, only for external threads, thread-rolling tools (cf. EMUGE manual, chapter 11, pages 373 to 404).
Cold-forming taps are threading tools with an approximately spirally or helically encircling thread profile, along which are arranged a plurality of pressing lobes (also referred to as shaping teeth, cold-forming teeth or shaping wedges) which are formed by generally rounded polygon corner regions, which are offset with respect to one another and project further outward, of an approximately polygonal cross section of the cold-forming tap. During the generation of the thread, the cold-forming tap is, similarly to the tap, moved with a feed motion axially with respect to the tool axis and whilst being rotated about its tool axis into a cylindrical core hole in a workpiece, wherein the tool axis of the tap is oriented coaxially with respect to the central axis of the core hole. The rotational speed and axial feed speed are coordinated with one another in accordance with the thread pitch. The pressing lobes of the cold-forming tap are permanently in engagement with the workpiece at the core hole wall, and press the thread flight into the core hole wall by plastic deformation, such that a continuous thread flight is generated on the core hole wall.
Furthermore, combination tools which operate exclusively by cutting processes and which are composed of drills and thread milling cutters are known, specifically so-called drill thread milling cutters (cf. EMUGE manual, chapter 10, page 354) and so-called circular drill thread milling cutters (cf. EMUGE manual, chapter 10, page 355), by means of which firstly the core hole for the thread can be generated, and then the thread can be generated in the core hole.
The tool shank of the specified thread generating tools is generally of at least approximately cylindrical form about its longitudinal axis and/or is received and held with its end facing away from the workpiece in the chuck of a machine tool. The direction of rotation of taps and cold-forming taps during the generation of the thread corresponds to the turning direction of the thread to be generated. The known screws or screw threads screwed into the generated internal thread comprise continuous helical external threads which are complementary to the internal threads.
DE 1 176 450 discloses a method for producing internal threads in sheet metal or similar workpieces, said method comprising the following method steps:
1. punching out a hole which is round per se and has a plurality of uniformly distributed cutouts,
2. inserting a tap, the number of cutting ridges of which corresponds to the number of cutouts,
3. rotating the tap through an angle which corresponds to the angle between two cutouts,
4. pulling out the tap.
In this known method, as indicated in DE 1 176 450, a tap, a non-cutting tap or a similar tool produces the thread by only partial rotation. When using a composite tool, it is possible in accordance with DE 1 176 450 to already form these internal threads as the part is being punched, it being possible for a plurality of internal threads to be produced at the same time even with different outer diameters and pitches. Production by a separate operation can be performed manually using a punching tool or a simple apparatus.
U.S. Pat. No. 3,359,581 discloses a method for producing threads in which a workpiece has one or two axial grooves and a thread generating tool has axial thread generating regions which are inserted coaxially into the grooves of the workpiece. Then, a thread is generated in the workpiece between the grooves by rotation through one turn in the case of one groove and through a half turn in the case of two grooves with the thread generating regions of the thread generating tool, and the thread generating tool is then axially removed again from the workpiece. The workpiece can have a core hole with two diametrically protruding grooves or else can be a cylindrical bolt with two diametric grooves. In the first case, the thread generating tool is a tap or thread-cutting tool having two thread-cutting ridges running axially diametrically opposite one another, and in the second case is a thread-cutting tool similar to a thread-cutting die having axially opposing, inwardly protruding thread-cutting ridges.
EP 2 218 536 A1 discloses a method for forming an internal thread on a main body, wherein firstly a threading tool having at least one portion with thread-forming means in certain regions is inserted into an opening in the main body, and then the entire internal thread is formed in the main body by a rotation of the threading tool through at most 360°. The threading tool has a plurality of portions with thread-forming means arranged in a symmetry of rotation of an integral order, and the threading tool is rotated through an angle of rotation which corresponds to the value of the quotient of a full circle divided by the number of portions with the thread-forming means of the threading tool. Before the threading tool is inserted, provision is made of recesses which are arranged in the inner wall of the main body in a symmetry of rotation of an integral order, the number of said recesses corresponding to the number of portions with the thread-forming means on the threading tool and the design of said recesses being coordinated with the design of the portions with the thread-forming means on the threading tool. In particular, provision is made of four recesses and four corresponding thread ridges on the threading tool.